PH adjustment of a melt for use in microetching glass substrates

ABSTRACT

A method of adjusting the pH of a strengthening melt to provide an adjusted melt for use in microetching glass substrates, such as glass disk substrates for use in data storage devices. A base is added to the strengthening melt to raise its pH. A desired degree of microetch is provided on an aluminosilicate glass disk substrate, for example, by immersion for 2-4 hours at 360° C. in a melt adjusted to have a pH of 10. This single operation both strengthens and microetches the glass substrate. A slight etching of the surface of a glass substrate, i.e., microetching, improves the performance and durability of a data storage disk made from the substrate. To avoid an overly aggressive etch that can create undesirable damage to the substrate surface, an acid may be added to the melt if the pH is subsequently determined to have shifted to above an upper limit.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This patent application is related to copending application Ser.No. 10/024,693 (docket no. ROC920010251US1), filed Dec. 18, 2001,entitled “PH ADJUSTMENT OF A STRENGTHENING MELT FOR USE IN STRENGTHENINGGLASS SUBSTRATES”, which is assigned to the assignee of the instantapplication.

FIELD OF THE INVENTION

[0002] The present invention relates in general to microetching glasssubstrates. More particularly, the present invention relates to a methodof adjusting the pH of a strengthening melt to provide an adjusted meltfor use in microetching glass substrates, such as glass disk substratesfor use in data storage devices.

BACKGROUND

[0003] A typical data storage device includes a medium for storing data,typically in magnetic, magneto-optical or optical form, and a transducerused to write and read data respectively to and from the medium. A diskdrive data storage device, for example, includes one or more datastorage disks coaxially mounted on a hub of a spindle motor. The spindlemotor rotates the data storage disks at speeds typically on the order ofseveral thousand or more revolutions-per-minute. Digital information,representing various types of data, is typically written to and readfrom the data storage disks by one or more transducers, or read/writeheads, which are mounted to an actuator assembly and passed over thesurface of the rapidly rotating disks.

[0004] In a typical magnetic disk drive, for example, data is stored ona magnetic layer coated on a disk substrate. Several characteristics ofdisk substrates significantly affect the areal density of a disk drive.One such characteristic that significantly affects the areal density ofa disk drive is the uniformity of the surface of the disk substrate,i.e., the absence of substrate surface defects. It is generallyrecognized that minimizing the flyheight, i.e., the clearance distancebetween the read/write head and the surface of a data storage disk,generally provides for increased areal densities. It is also recognizedin the art, however, that the smoothness of the surface of a datastorage disk becomes a critical factor and design constraint whenattempting to minimize the flyheight. A significant decrease inflyheight provided by the use of data storage disks having highlyuniform recording surfaces can advantageously result in increasedtransducer readback sensitivity and increased areal density of the diskdrive. The uniformity of disk substrate surfaces affects the uniformityof the recording surfaces because the layers sputtered onto the disksubstrate, such as the magnetic layer, replicate any irregular surfacemorphology of the disk substrate.

[0005] Conventionally, disk substrates have been based upon aluminum,such as NiP coated Al/Mg alloy substrates. Coating the aluminummagnesium alloy with a nickel-phosphorus plate provides a harderexterior surface which allows the disk substrate to be polished andsuperfinished. A conventional superfinishing process and slurry isdescribed in U.S. Pat. No. 6,236,542 to Hartog et al., which is assignedto the assignee of the present application. Typically, the Al/Mg—NiPsubstrate is superfinished to a smooth finish with a colloidal slurry,e.g., a pH adjusted aqueous slurry containing colloidal silica and/orcolloidal alumina particles and an etching agent such as a nitrate,prior to sputtering with thin film magnetic coatings. The colloidalslurry is then cleaned from the substrate by the general cleaningmechanisms of mechanical scrubbing, dispersion and etching. Surfactantsand pH are generally used for dispersion cleaning, where the surfactantand pH act to separate the slurry particles from each other and from thesubstrate. Etching is generally accomplished by acids and acid soapsthat erode or dissolve the substrate material beneath embedded slurryparticles (under-cut) to release them from the substrate. Typical acidsin use for NiP plated Al-based substrates include, for example, straightphosphoric acid, nitric acid, hydrofluoric acid-based soaps andphosphoric acid-based soaps. The straight acids generally have a pH lessthan 1 and the soaps generally have pH's above 1.

[0006] After cleaning, the substrates are sputtered with a series oflayers, e.g., a chrome underlayer, a magnetic layer and a carbonprotection layer. If residual slurry particles are left on the substrateor there is galling to the relatively soft NiP layer, the sputteredlayers replicate the irregular surface morphology, creating a bumpysurface on the finished disk. When the read/write head glides over thesurface, it crashes into bumps created by the residual particles and/ordamage that is higher than the glide clearance. This is known as a glidedefect, which can ultimately cause disk drive failure. These bumpsfurther cause magnetic defects, corrosion and decreased disk life. Thus,the residual slurry particles and/or damage needs to be removed from thepolished substrate surface so that the substrate is as smooth aspossible.

[0007] Unfortunately, aluminum-based substrates have relatively lowspecific stiffness, as well as relatively low impact and dentresistance. For example, the relatively low specific stiffness of theAl/Mg—NiP substrates (typically 3.8 Mpsi/gm/cc) makes this type of disksubstrate susceptible to environmental forces which create disk flutterand vibration and which may cause the read/write head to impact and dentthe disk substrate surface.

[0008] More recently, glass substrates have been used for disk drives inportable devices, such as laptop computers. Glass substrates have ahigher impact and dent resistance than aluminum-based substrates, whichis important in portable devices where the unit is subject to beingbumped, dropped and banged around, causing the read/write head to bangon the disk substrate surface. As discussed in more detail below, glasssubstrates are typically strengthened by immersion in a strengtheningmelt. Moreover, the specific stiffness of glass substrates (typically ≦6or 7 Mpsi/gm/cc) is typically higher than that of aluminum-basedsubstrates.

[0009] An additional benefit of glass is that it is easier to polish toand maintain as a smooth surface finish (as compared to NiP) thanaluminum-based substrates. A smoother substrate allows the read/writehead to fly closer to the disk, which produces a higher densityrecording. Glide height for some computer disk drives is on the order of20 nanometers (about 200 Å) and less, which is an extremely smallinterface distance. Thus, the fact that glass substrates can be polishedto smoother finishes makes an industry shift from Al-based substrates toglass substrates desirable, not only for disk drives used in portabledevices, but for disk drives used in stationary devices as well.

[0010] Just as with aluminum-based substrates, the surface of the glasssubstrate needs to be polished and superfinished with a slurry toprovide an atomically smooth surface. Such a conventional superfinishingpolish process and slurry is also described in the above referenced U.S.Pat. No. 6,236,542 to Hartog et al. Typically, the glass substrate issuperfinished to a smooth finish with a colloidal slurry, e.g., a pHadjusted aqueous slurry containing colloidal silica and/or colloidalalumina particles and an etching agent such as cerium sulfate, prior tostrengthening in a strengthening melt and sputtering with thin filmmagnetic coatings.

[0011] In this conventional superfinishing polish process colloidalsilica particles attach to the surface being polished not only by theusual London dispersion forces, van der Waals forces and hydrogenbonding, but unlike NiP, also by molecular bonding even though theslurry has the usual stabilizing agents used in the colloidal silica toprevent the silica particles from sticking to each other (interparticlesiloxane bonding), charge repulsion and/or steric stabilizers. Standardmethods of scrubbing with soaps using polyvinyl alcohol (PVA) pads,ultrasonics or megasonics will not remove any significant percentage ofsuch molecular bonded silica particles. Just as with aluminum-basedsubstrates, if these particles are left in place on the glass substrate,glide defects occur that can ultimately cause disk drive failure. Theseglide defects further cause magnetic defects, corrosion and decreaseddisk life.

[0012] A less-than-optimal solution to this problem is to use strongeracid or base solutions than the cleaning soap, to etch the glasssubstrate or undercut the slurry particles similar to what can be doneto remove hard alpha alumina from Al/Mg—NiP substrates afternon-superfinish polish slurries. However, the surface finish of glasssubstrates can be damaged by such a technique through surface topographychange such as pitting and chemical composition changes. Glass has lowresistance to acid etching and overly aggressive acid solutions, such ashydrofluoric acid, and caustic etching at high pH's and temperatures.The damage to the superfinished glass surface may be sufficient enoughto adversely affect the morphology of layers deposited by subsequentsputtering processes and can cause magnetic, glide and corrosionfailures.

[0013] A better solution to this problem is to use a cleaning polishetch solution/process (a process performed by running disk substrates ona polishing pad using an etch solution instead of a slurry, i.e., thereare no slurry particles in the cleaning polish etch solution) with acid,neutral or base solutions to etch the glass substrate and/or theattached slurry particles under polish conditions thereby maintainingthe superfinish surface while removing the superfinish polish slurrydebris by etching and dilution. Such a cleaning polish etchsolution/process is disclosed in the copending application Ser. No.09/976,412 (docket no. ROC920010283US1) entitled “CLEANING POLISH ETCHCOMPOSITION AND PROCESS FOR A SUPERFINISHED SURFACE OF A SUBSTRATE”,assigned to the same assignee as the present application. Etching byitself (i.e., the first solution discussed above) with PVA scrub,ultrasonics or megasonics is what has been done to remove slurryparticles from Al/Mg—NiP or glass substrates, but with the less than 20nm glide heights now in use, a cleaning polish etch solution/processensures 100% surface cleaning of particles that small (i.e., the lowerthe glide height, the smaller the particles needing to be removed, andthus the more difficult they are to remove) while maintaining thesurface finish. The cleaning polish etch process, however, addsequipment and handling costs. Nonetheless, without the cleaning polishetch process the surface of the glass substrate can be damaged by usingonly chemical etch due to the low resistance of the glass material toacid etching or overly aggressive caustic etch solutions.

[0014] An even better solution to this problem is to use a self-cleaningcolloidal slurry and process, such as disclosed in the copendingapplication Ser. No. 09/976,167 (docket no. ROC920010111US1) entitled“SELF-CLEANING COLLOIDAL SLURRY COMPOSITION AND PROCESS FOR FINISHING ASURFACE OF A SUBSTRATE”, assigned to the same assignee as the presentapplication. The slurry comprises a carrying fluid, colloidal particles,etchant, and a surfactant adsorbed and/or precipitated onto a surface ofthe colloidal particles and/or substrate. The surfactant has ahydrophobic section that forms a steric hindrance barrier andsubstantially prevents contaminates, including colloidal particles, frombonding to the substrate surface. Subsequent cleaning with standard soapsolutions removes substantially all remaining contamination from thesubstrate surface.

[0015] After cleaning, the glass substrate is typically subjected tochemical strengthening. Chemical strengthening is known in the art oftreating glass. In chemical strengthening, the substrate is immersed ina strengthening melt, e.g., molten potassium nitrate and/or sodiumnitrate, typically for at least 1 hour to strengthen the glass againstbreaking. In the strengthening melt, an ion exchange process strengthensthe glass substrate by exchanging smaller ions near the substratesurface for larger ions of the strengthening melt below thetransformation temperature of the glass to generate pressure stresszones at the substrate surface.

[0016] It is known that by slightly etching, or microetching, thesurface of glass disk substrates, the performance and durability of datastorage disks made therefrom can be improved. Microetching isconventionally accomplished by immersing the glass disk substrates in astrong acid bath, e.g., a hydrofluoric (HF) acid bath, typically afterthe substrates have been superfinished, cleaned and strengthened.Unfortunately, immersion of substrates in strong acid baths involvessafety risks and additional process steps. Moreover, the surface finishof glass substrates can be damaged by techniques such as this thatemploy strong acid or base solutions. The damage can include surfacetopography change such as pitting and chemical composition changes.Glass has low resistance to acid etching and overly aggressive acidsolutions, such as HF acid, and caustic etching at high pH's andtemperatures. The damage to the superfinished glass surface may besufficient enough to adversely affect the morphology of layers depositedby subsequent sputtering processes and can cause magnetic, glide andcorrosion failures.

[0017] If the market trend toward glass substrates in disk drives is tosucceed, an enhanced mechanism for microetching glass substrates isrequired. Preferably, such an enhanced mechanism would not involveadditional process steps and safety risks. Also, such an enhancedmechanism would preferably not cause undesirable damage to asuperfinished surface of a glass disk substrate.

SUMMARY OF THE INVENTION

[0018] An object of the present invention is to provide an enhancedmechanism for microetching glass substrates.

[0019] Another object of the present invention is to provide such anenhanced mechanism for microetching glass substrates that does notinvolve additional process steps and safety risks.

[0020] Yet another object of the present invention is to provide such anenhanced microetching mechanism that does not cause undesirable damageto a superfinished surface of a glass disk substrate.

[0021] These and other objects of the present invention are achieved bya method of adjusting the pH of a strengthening melt (e.g., moltenpotassium nitrate and/or sodium nitrate) to provide an adjusted melt foruse in microetching glass substrates, such as glass disk substrates foruse in data storage devices. A base (e.g., sodium hydroxide) is added tothe strengthening melt to raise the pH to a level at which microetchingoccurs in aqueous solution for a given glass (e.g., a pH of about 9 to11 for silicate glass). Preferably, a glass substrate is immersed in theadjusted melt for a time sufficient to provide a desired degree ofmicroetch (e.g., about 1 to 24 or more hours for silicate glass) withthe temperature of the adjusted melt being below the strain point, ortransformation temperature, of the glass (e.g., about 280° C. to 420° C.for silicate glass in molten potassium nitrate and/or sodium nitrate).More preferably, the glass substrate is aluminosilicate, the adjustedmelt has a pH of about 10, and the glass substrate is immersed in theadjusted melt for about 2 to 4 hours at about 360° C. This singleimmersion operation both strengthens and microetches the glasssubstrate, without additional process steps and safety concerns. Aslight etching of the surface of a glass substrate, i.e., microetching,improves the performance and durability of a data storage disk made fromthe glass substrate. For glass disk substrates, it is generallypreferred that the microetching produce an overall surfacemicroroughness (R_(q)) within the range of about 5 Å to about 7 Å. Toavoid an overly aggressive etch that can create undesirable damage tothe substrate surface, an acid (e.g., nitric acid) may be added to theadjusted melt if the pH is subsequently determined to have shifted toabove an upper limit (e.g., an upper pH limit of about 11 for silicateglass).

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The present invention together with the above and other objectsand advantages can best be understood from the following detaileddescription of the embodiments of the invention illustrated in thedrawings, wherein like reference numerals denote like elements.

[0023]FIG. 1 is a top view of a data storage device with its upperhousing cover removed and employing one or more data storage diskshaving glass disk substrates that have been microetched with a pHadjusted melt in accordance with the present invention.

[0024]FIG. 2 is a side plan view of a data storage device comprising aplurality of data storage disks having glass disk substrates that havebeen microetched with a pH adjusted melt in accordance with the presentinvention.

[0025]FIG. 3 is a perspective view of a disk substrate that has beenmicroetched with a pH adjusted melt in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Overview

[0026] The present invention utilizes a base to adjust the pH of astrengthening melt to provide an adjusted melt for use in microetchingglass substrates, such as glass disk substrates for use in data storagedevices. The strengthening melt may be molten potassium nitrate and/orsodium nitrate, for example. A base (e.g., sodium hydroxide) is added tothe strengthening melt to raise the pH to a level at which microetchingoccurs in aqueous solution for a given glass (e.g., a pH of about 9 to11 for silicate glass). Preferably, a glass substrate is immersed in theadjusted melt for a time sufficient to provide a desired degree ofmicroetch (e.g., about 1 to 24 or more hours for silicate glass) whilethe temperature of the adjusted melt is maintained below the strainpoint, or transformation temperature, of the glass (e.g., about 280° C.to 420° C. for silicate glass in molten potassium nitrate and/or sodiumnitrate). More preferably, the glass substrate is aluminosilicate, theadjusted melt has a pH of about 10, and the glass substrate is immersedin the adjusted melt for about 2 to 4 hours while the temperature ismaintained at about 360° C. This single immersion operation bothstrengthens and microetches the glass substrate, without additionalprocess steps and safety concerns. A slight etching of the surface of aglass substrate, i.e., microetching, improves the performance anddurability of a data storage disk made from the glass substrate. Forglass disk substrates, it is generally preferred that the microetchingproduce an overall surface microroughness (R_(q)) within the range ofabout 5 Å to about 7 Å. This degree of microetching typically addsaerodynamic stability to the head/disk interface, reducing slidermodulation and head crashes, and improves off-track error rate. To avoidan overly aggressive etch that can create undesirable damage to thesubstrate surface, an acid (e.g., nitric acid) may be added to theadjusted melt if the pH is subsequently determined to have shifted toabove an upper limit (e.g., an upper pH limit of about 11 for silicateglass).

[0027] In an exemplary embodiment that is described in greater detailbelow, an adjusted melt was made by adding sodium hydroxide to aconventional strengthening melt comprising potassium nitrate and sodiumnitrate to raise the pH from an initial pH of 7.0 to an adjusted pH of10.2. Superfinished aluminosilicate glass disk substrates were bothstrengthened and microetched by immersion in the pH adjusted melt(pH=10.2) at 360° C. for 3 hours. The surface microetch was uniform andfree from high spots or peaks as seen by atomic force microscope (AFM).The overall (root mean square) surface microroughness (R_(q)) was 5.23Å, the average height of peaks above average roughness (R_(p)) was 28 Å,and the difference between the highest peak and the lowest valley(R_(max)) was 45 Å. A data storage disk having improved performance anddurability can be provided by applying a recording layer over themicroetched surface of the aluminosilicate glass disk substrate treatedwith the pH adjusted melt (pH=10.2) as compared to the conventionalstrengthening melt (pH=7.0).

The Data Storage Device

[0028] Referring now to the drawings, and more particularly to FIGS. 1and 2, there is shown a magnetic data storage device 20 utilizingmagnetic disks with disk substrates that have been microetched using apH adjusted melt in accordance with the preferred embodiment of thepresent invention. Magnetic data storage device 20 is shown in FIG. 1with its cover (not shown) removed from a base 22 of a housing 21. Asbest seen in FIG. 2, the magnetic data storage device 20 includes one ormore rigid data storage disks 24 that are rotated by a spindle motor 26.The rigid data storage disks 24 are constructed with a disk substrateupon which a recording layer is formed. In an exemplary construction, amagnetizable recording layer is formed on a glass disk substrate.Alternatively, an optical recording layer or a magneto-optical recordinglayer may be formed on the disk substrate in lieu of the magnetizablerecording layer.

[0029] Referring back to FIG. 1, an actuator assembly 37 typicallyincludes a plurality of interleaved actuator arms 30, with each armhaving one or more suspensions 28 and transducers 27 mounted onairbearing sliders 29. The transducers 27 typically include componentsboth for reading and writing information to and from the data storagedisks 24. Each transducer 27 may be, for example, a magnetoresistive(MR) head having a write element and a MR read element. Alternatively,each transducer may be an inductive head having a combined read/writeelement or separate read and write elements, or an optical head havingseparate or combined read and write elements. The actuator assembly 37includes a coil assembly 36 which cooperates with a permanent magnetstructure 38 to operate as an actuator voice coil motor (VCM) 39responsive to control signals produced by a controller 58. Thecontroller 58 preferably includes control circuitry that coordinates thetransfer of data to and from the data storage disks 24, and cooperateswith the VCM 39 to move the actuator arms 30 and suspensions 28, toposition transducers 27 to prescribed track 50 and sector 52 locationswhen reading and writing data from and to the data storage disks 24.

The Disk Substrate

[0030]FIG. 3 shows a disk substrate that has been microetched using a pHadjusted melt in accordance with the preferred embodiment of the presentinvention. Disk substrate 300, which has a disk substrate surface 302,is preferably a material having a relatively high specific stiffness(e.g., ≧3.8 Mpsi/gm/cc) such as a glass or glass-ceramic. Morepreferably, the disk substrate 300 is an aluminosilicate glass. A commonsubstrate material, e.g., aluminosilicate glass, has been chosen for thepreferred embodiment to best illustrate the teachings of the presentinvention. However, it should be understood that the present inventionis not limited to just aluminosilicate glass. Other glass-basedsubstrate materials including other types of glass, such as silica sodalime glass, and glass-ceramics may be used. These glass-based substratesare hereinafter referred to as glass substrates.

[0031] A representative list of compositions along with their relativespecific stiffnesses (Mpsi/gm/cc) that may be used is found in Table 1below. TABLE 1 Specific Material Stiffness Aluminosilicate glass 4.9Lithium silicate glass 5.2 Canasite glass ceramic 4.6 Flint glassceramic 6.6 Quartz glass 4.9-6.1

[0032] These materials may be used alone or in combination to providethe disk substrate of the appropriate stiffness. Preferably, the disksubstrate has a stiffness of at least about 3.8 Mpsi/gm/cc.

[0033] Glass is generally a silicate material having a structure ofsilicon and oxygen where the silicon atom is tetrahedrally coordinatedto surrounding oxygen atoms. Any number of materials may be used to formglass such as boron oxide, silicon oxide, germanium oxide, aluminumoxide, phosphorous oxide, vanadium oxide, arsenic oxide, antimony oxide,zirconium oxide, titanium oxide, aluminum oxide, thorium oxide,beryllium oxide, cadmium oxide, scandium oxide, lanthanum oxide, yttriumoxide, tin oxide, gallium oxide, indium oxide, lead oxide, magnesiumoxide, lithium oxide, zinc oxide, barium oxide, calcium oxide, stroniumoxide, sodium oxide, cadmium oxide, potassium oxide, rubidium oxide,mercury oxide, and cesium oxide.

[0034] Glass-ceramics may also be used for the disk substrate.Glass-ceramics generally result from the melt formation of glass andceramic materials by conventional glass manufacturing techniques.Subsequently, the materials are heat cycled to cause crystallization.Typical glass-ceramics are, for example, β-quartz solid solution, SiO₂;β-quartz; lithium metasilicate, Li₂O—SiO₂; lithium disilicate, Li₂(SiO₂)₂; β-spodumene solid solution; anatase, TiO₂; β-spodumene solidsolution; rutile TiO₂; β-spodumene solid solution; mullite,3Al₂O₃—2SiO₂; β-spodumene dorierite, 2MgO—2Al₂O₃—5SiO₂; spinel,MgO—Al₂O₃; MgO-stuffed; β-quartz; quartz; SiO₂; alpha-quartz solidsolution, SiO₂; spinel, MgO—Al₂O₃; enstatite, MgO—SiO₂; fluorphlogopitesolid solution, KMg₃AlSi₃O₁₀F₂; mullite, 3Al₂O₃—2SiO₂; and (Ba, Sr,Pb)Nb₂O₆.

[0035] The disk substrate may be made entirely of one material, or mayinclude a coating layer applied over at least one surface of an innercore.

[0036] Also, it should be understood that the present invention is notlimited to disk substrates. The present invention is equally applicableto other applications that involve microetching a glass substrate. Forexample, the present invention may be utilized in applications such aslens fabrication and mirror fabrication.

Polishing/Superfinishing and Cleaning the Disk Substrate

[0037] Just as with aluminum-based substrates, the surface of the glasssubstrate needs to be polished and superfinished with a slurry toprovide an atomically smooth surface. Such a conventional superfinishingpolish process and slurry is described in U.S. Pat. No. 6,236,542 toHartog et al. Typically, the glass substrate is superfinished to asmooth finish with a colloidal slurry, e.g., a pH adjusted aqueousslurry containing colloidal silica and/or colloidal alumina particlesand an etching agent such as cerium sulfate, prior to strengthening in astrengthening melt and then sputtering with thin film magnetic coatings.

[0038] In this conventional superfinishing polish process colloidalsilica particles attach to the surface being polished not only by theusual London dispersion forces, van der Waals forces and hydrogenbonding, but unlike NiP, also by molecular bonding even though theslurry has the usual stabilizing agents used in the colloidal silica toprevent the silica particles from sticking to each other (interparticlesiloxane bonding), charge repulsion and/or steric stabilizers. Standardmethods of scrubbing with soaps using polyvinyl alcohol (PVA) pads,ultrasonics or megasonics will not remove any significant percentage ofsuch molecular bonded silica particles. Just as with aluminum-basedsubstrates, if these particles are left in place on the glass substrate,glide defects occur that can ultimately cause disk drive failure. Theseglide defects further cause magnetic defects, corrosion and decreaseddisk life.

[0039] A less-than-optimal solution to this problem is to use strongeracid or base solutions than the cleaning soap, to etch the glasssubstrate or undercut the slurry particles similar to what can be doneto remove hard alpha alumina from Al/Mg—NiP substrates afternon-superfinish polish slurries. However, the surface finish of glasssubstrates can be damaged by such a technique through surface topographychange such as pitting and chemical composition changes. Glass has lowresistance to acid etching and overly aggressive acid solutions, such ashydrofluoric acid, and caustic etching at high pH's and temperatures.The damage to the superfinished glass surface may be sufficient enoughto adversely affect the morphology of layers deposited by subsequentsputtering processes and can cause magnetic, glide and corrosionfailures.

[0040] A better solution to this problem is to use a cleaning polishetch solution/process (a process performed by running disk substrates ona polishing pad using an etch solution instead of a slurry, i.e., thereare no slurry particles in the cleaning polish etch solution) with acid,neutral or base solutions to etch the glass substrate and/or theattached slurry particles under polish conditions thereby maintainingthe superfinish surface while removing the superfinish polish slurrydebris by etching and dilution. Such a cleaning polish etchsolution/process is disclosed in the copending application Ser. No.09/976,412 (docket no. ROC920010283US1) entitled “CLEANING POLISH ETCHCOMPOSITION AND PROCESS FOR A SUPERFINISHED SURFACE OF A SUBSTRATE”,assigned to the same assignee as the present application. Etching byitself (i.e., the first solution discussed above) with PVA scrub,ultrasonics or megasonics is what has been done to remove slurryparticles from Al/Mg—NiP or glass substrates, but with the less than 20nm glide heights now in use, a cleaning polish etch solution/processensures 100% surface cleaning of particles that small (i.e., the lowerthe glide height, the smaller the particles needing to be removed, andthus the more difficult they are to remove) while maintaining thesurface finish. The cleaning polish etch process, however, addsequipment and handling costs. Nonetheless, without the cleaning polishetch process the surface of the glass substrate can be damaged by usingonly chemical etch due to the low resistance of the glass material toacid etching or overly aggressive caustic etch solutions.

[0041] An even better solution to this problem is to use a self-cleaningcolloidal slurry and process, such as disclosed in the copendingapplication Ser. No. 09/976,167 (docket no. ROC920010111US1) entitled“SELF-CLEANING COLLOIDAL SLURRY COMPOSITION AND PROCESS FOR FINISHING ASURFACE OF A SUBSTRATE”, assigned to the same assignee as the presentapplication. The slurry comprises a carrying fluid, colloidal particles,etchant, and a surfactant adsorbed and/or precipitated onto a surface ofthe colloidal particles and/or substrate. The surfactant has ahydrophobic section that forms a steric hindrance barrier andsubstantially prevents contaminates, including colloidal particles, frombonding to the substrate surface. Subsequent cleaning with standard soapsolutions removes substantially all remaining contamination from thesubstrate surface.

[0042] Any conventional polishing and/or superfinishing processes andslurry particle removal techniques may be used to prepare the glasssubstrate for immersion in the pH adjusted melt (and/or subsequent toimmersion in the pH adjusted melt), and the present invention is neitherlimited to the superfinishing processes and cleaning techniquesdiscussed above nor the sequence of those methods and techniquesrelative to immersion of the glass substrate in the pH adjusted melt.

Treatment of the Disk Substrate in the pH Adjusted Melt

[0043] The present invention utilizes a base to adjust the pH of astrengthening melt to provide an adjusted melt for use in microetchingglass substrates, such as glass disk substrates for use in data storagedevices. Strengthening melts are typically nitrates such as potassiumnitrate (KNO₃) and/or sodium nitrate (NaNO₃). However, otherstrengthening melts may be used in lieu of, or in addition to KNO₃and/or NaNO₃. Examples of strengthening melts that may be usedconsistent with the present invention include KNO₃, NaNO₃, AgNO₃,K₂Cr₂O₇, Na₂Cr₂O₇, and the like, and combinations thereof.

[0044] An appropriate amount of a base is added to the strengtheningmelt to raise the pH of the adjusted melt to within a predetermined pHrange. The predetermined pH range is selected so that the adjusted meltwill slightly etch the surface of a glass substrate immersed therein. Aslight etching of the surface of a glass substrate, i.e., microetching,improves the performance and durability of a data storage disk made fromthe glass substrate. For glass disk substrates, it is generallypreferred that the microetching produce an overall surfacemicroroughness (R_(q)) within the range of about 5 Å to about 7 Å.

[0045] The microetching process of the present invention generatesdesirable surface roughness that enhances the aerodynamics andadhesion/bonding of layers (e.g., a recording layer) subsequentlydeposited on the glass substrate. The aerodynamic improvement reducesoff-track pull due to radial forces acting on the head and improves headflying stability. Minor and controllable etching of the glass substratesurface occurs when the pH of the strengthening melt is within thepredetermined pH range. However, when the pH of the strengthening meltis increased to beyond an upper limit of the predetermined pH range anoverly aggressive etch occurs, causing undesirable damage (e.g., surfacetopographical change such as pitting and chemical compositional change)to the superfinished surface that may be sufficient enough to adverselyaffect the morphology of layers deposited by subsequent processes andcan cause magnetic, glide and corrosion failures.

[0046] For microetching silicate glass, for example, the predeterminedpH range of the adjusted melt typically includes values above and below10. More particularly, the predetermined pH range for microetchingsilicate glass is preferably 9 to 11, and more preferably 9.5 to 10.5.The base is preferably added while the strengthening melt is in a moltenstate and is preferably selected to avoid particle formation when addedto the strengthening melt. Sodium or potassium hydroxide, for example,is non-particle-forming with respect to nitrate based strengtheningmelts such as potassium nitrate and/or sodium nitrate. Other bases maybe used in lieu of, or in addition to sodium and potassium hydroxide.Useful non-particle-forming bases generally include inorganic bases suchas lithium hydroxide, sodium hydroxide, potassium hydroxide, calciumhydroxide, magnesium hydroxide, silver (I) oxide, and combinationsthereof. In any event, the base is preferably selected to avoid particleformation in the strengthening melt.

[0047] Preferably, the glass substrate is immersed in the adjusted meltfor a time sufficient to provide a desired degree of microetch (e.g.,about 1 to 24 or more hours for silicate glass) while the temperature ofthe adjusted melt is maintained below the strain point, ortransformation temperature, of the glass (e.g., about 280° C. to 420° C.for silicate glass in molten potassium nitrate and/or sodium nitrate).More preferably, the glass substrate is aluminosilicate, the adjustedmelt has a pH of about 10, and the glass substrate is immersed in theadjusted melt for about 2 to 4 hours while the temperature is maintainedat about 360° C. This single immersion operation both strengthens andmicroetches the glass substrate, without additional process steps andsafety concerns. A slight etching of the surface of a glass substrate,i.e., microetching, improves the performance and durability of a datastorage disk made from the glass substrate. Microetching typically addsaerodynamic stability to the head/disk interface, reducing slidermodulation and head crashes, and improves off-track error rate. To avoidan overly aggressive etch that can create undesirable damage to thesubstrate surface, an acid (e.g., nitric acid) may be added to theadjusted melt if the pH is subsequently determined to have shifted toabove an upper limit (e.g., an upper pH limit of about 11 for silicateglass).

[0048] Strengthening melts are subject to pH shift that can cause glasssubstrates strengthened therein to aggressively etch, creating angstromto nanometer size pits on the surface of the glass substrates. The pHshift can come from sources such as the thermal decomposition of thestrengthening melt, the glass substrates themselves (typically, alkaliglass), and/or incoming salts with high pH. Typically, the pH shiftworsens with repeated use of the strengthening melt to treat more andmore glass substrates.

[0049] An appropriate amount of an acid may be added if the pH of theadjusted melt rises above a predetermined upper pH limit. The upper pHlimit is selected so that the acid will neutralize the salt bath andsubstantially eliminate aggressive etching. For microetching silicateglass, for example, the upper pH limit is preferably 11, and morepreferably 10.5. In addition to pH shift, it may be necessary to add anacid to the adjusted melt if too much base is added. The acid ispreferably added while the adjusted melt is in a molten state andpreferably selected to avoid particle formation when added to theadjusted melt. Nitric acid, for example, is non-particle-forming withrespect to nitrate based melts such as potassium nitrate and/or sodiumnitrate. The present invention is not limited to the use of nitric acidas the non-particle-forming acid, however. Other non-particle-formingacids may be used in lieu of, or in addition to nitric acid. Usefulnon-particle-forming acids generally include acids such as nitric acidand chromic acid, and combinations thereof. In any event, the acid ispreferably selected to avoid particle formation when added to theadjusted melt.

[0050] For nitrate-based molten salt baths, an advantage to using nitricacid (HNO₃) as the non-particle-forming acid and potassium hydroxide(KOH) and/or sodium hydroxide (NaOH) as the non-particle-forming base isthat the neutralization products are potassium nitrate (KNO₃) and sodiumnitrate (NaNO₃), which are typically constituents in the molten saltbath. Similarly, for dichromate-based molten salt baths, an advantage tousing chromic acid (CrO₃) as the non-particle-forming acid and potassiumhydroxide (KOH) and/or sodium hydroxide (NaOH) as thenon-particle-forming base is that the neutralization products arepotassium dichromate (K₂Cr₂O₇) and sodium dichromate (Na₂Cr₂O₇), whichare typically constituents in the molten salt bath. Also, any otheralkali species in the molten salt bath will be neutralized into ionicnitrate or dichromate compounds, which typically comprise 100% of themolten salt bath. The neutralization products are generally classicacid/base salts—in this particular case, nitrated ionic or dichromatedionic compounds.

[0051] The appropriate amount of acid or base to be added to the melt ispreferably determined based on the pH of the melt. Preferably, theappropriate amount of acid or base is added to maintain the pH of themelt in a predetermined pH range—which for silicate glass is preferably9 to 11, and more preferably 9.5 to 10.5.

[0052] The pH of the melt may be determined using conventionaltechniques. Because the melt is not an aqueous environment, its pH istypically not directly measured using a pH meter, for example. Instead,the pH of the melt may be determined using other conventional techniquessuch as by an indirect measure of the caustic material (e.g., sodiumoxide, lithium oxide, potassium oxide and the like leached from theglass substrate) in the melt by titration against a reagent of knownmolarity. Such conventional techniques of pH determination are wellknown in the art, and thus are not further discussed herein.

[0053] Given the molten state of the melt as the acid or base is added,the addition preferably occurs slowly and near the bottom of the melt.For example, a metering pump may be used to control the rate at whichthe acid or base is added to the melt through a dispersive dischargepositioned at the bottom of the melt vessel.

[0054] The ability to raise the pH of the melt was demonstrated byadding 10 gm of sodium hydroxide (NaOH) to a melt comprised of a 30pound molten salt mixture (approximately 60/40) of potassium nitrate(KNO₃) and sodium nitrate (NaNO₃). In this case, the melt was at aninitial pH of 8.77 and was raised to a pH of 10.5.

[0055] The ability to lower the pH of the melt was also demonstrated byadding 5.0 ml of nitric acid (HNO₃) to a melt comprised of a 30 poundmolten salt mixture (approximately 60/40) of potassium nitrate (KNO₃)and sodium nitrate (NaNO₃). In this case, the melt was at an initial pHof 9.1 and was lowered to a pH of 6.5.

[0056] The pH of the melt may be determined on a periodic basis (e.g.,after a predetermined number of glass substrates have been processedand/or after a predetermined period of time the melt has been heated),or after a specific event (e.g., after a temperature excursion and/orafter material is added to the melt).

[0057] Alternatively, the appropriate amount of acid or base to be addedto the melt may be determined without actual determination of the pH ofthe melt. For example, the pH of the melt may be estimated (without anactual determination) based on process history criteria such as thenumber of glass substrates that have been processed and/or the period oftime the melt has been heated.

[0058] When the melt is adjusted within the predetermined pH range, thedegree of etching may be controlled by the temperature of the adjustedmelt and/or the duration of immersion. Silicate glass disk substrates,for example, are preferably immersed in the pH adjusted melt for 1 to 24or more hours while the temperature is maintained at about 280° C. to420° C., more preferably about 2 to 4 hours while the temperature ismaintained at about 360° C. The surface etch of glass disk substratestreated in the pH adjusted melt was uniform and free from high spots orpeaks as seen by atomic force microscope (AFM).

EXAMPLE

[0059] Sodium hydroxide (NaOH) was added to a conventional strengtheningmelt comprised of a 30 pound molten salt bath mixture of 60% potassiumnitrate (KNO₃) and 40% sodium nitrate (NaNO₃) to increase the pH from aninitial pH of 7.0 to an adjusted pH of 10.2. About 10 gm of sodiumhydroxide was added. Superfinished aluminosilicate glass disk substrateswere treated in both the conventional strengthening melt (pH=7.0) andthe pH adjusted melt (pH=10.2) at 360° C. for 3 hours. Superfinishedaluminosilicate glass disk substrates treated in the conventionalstrengthening melt (pH=7.0) strengthening melt were strengthened, butnot microetched. In contrast, superfinished aluminosilicate glass disksubstrates treated in the pH adjusted melt (pH=10.2) were bothstrengthened and microetched. The surface microetch was uniform and freefrom high spots or peaks as seen by atomic force microscope (AFM).Aluminosilicate glass disk substrates strengthened and microetched inthe pH adjusted melt (pH=10.2) had surface roughness parameters ofR_(q)/R_(p)/R_(max) of 5.23 Å/28 Å/45 Å. Aluminosilicate glass disksubstrates strengthened in the conventional strengthening melt (pH=7.0)had surface roughness parameters of R_(q)/R_(p)/R_(max) of 3.94 Å/21Å/54 Å. A data storage disk having improved performance and durabilitycan be provided by applying a recording layer over the microetchedsurface of the aluminosilicate glass disk substrate treated with the pHadjusted melt (pH=10.2) as compared to the conventional strengtheningmelt (pH=7.0).

[0060] While this invention has been described with respect to thepreferred and alternative embodiments, it will be understood by thoseskilled in the art that various changes in detail may be made thereinwithout departing from the spirit, scope, and teaching of the invention.For example, the invention may be utilized in other data storage mediumapplications, such as in optical storage medium applications.Additionally, the invention may be utilized in applications other thandata storage device applications, such as in lens fabricationapplications, mirror fabrication applications or other applications thatinvolve microetching a glass substrate. Accordingly, the hereindisclosed invention is to be limited only as specified in the followingclaims.

What is claimed is:
 1. A method of microetching glass disk substratesfor use in storage devices, comprising the steps of: providing astrengthening melt; providing an adjusted melt by adding a base to thestrengthening melt such that the pH of the adjusted melt is sufficientfor microetching a selected glass composition; placing a glass disksubstrate having the selected glass composition in the adjusted melt. 2.The method as recited in claim 1, wherein the strengthening meltcomprises at least one of potassium nitrate, sodium nitrate, silvernitrate, potassium dichromate and sodium dichromate; and wherein thebase comprises at least one of sodium hydroxide, potassium hydroxide,calcium hydroxide, magnesium hydroxide, lithium hydroxide and silver (I)oxide.
 3. The method as recited in claim 2, wherein the strengtheningmelt comprises at least one of potassium nitrate and sodium nitrate; andwherein the base comprises at least one of sodium hydroxide andpotassium hydroxide.
 4. The method as recited in claim 3, wherein theglass disk substrate is an aluminosilicate glass disk substrate, the pHof the adjusted melt is no less than about 10, the temperature of theadjusted melt is about 360° C., and the aluminosilicate glass disksubstrate is placed in the adjusted melt for between about 2 and 4hours.
 5. The method as recited in claim 1, wherein the glass disksubstrate is a silicate glass disk substrate, and further comprising thesteps of: determining the pH of the adjusted melt; adding an acid to theadjusted melt if the pH is determined to be above about
 11. 6. Themethod as recited in claim 5, wherein the strengthening melt comprisesat least one of potassium nitrate, sodium nitrate, silver nitrate,potassium dichromate and sodium dichromate; and wherein the basecomprises at least one of sodium hydroxide, potassium hydroxide, calciumhydroxide, magnesium hydroxide, lithium hydroxide and silver (I) oxide.7. The method as recited in claim 6, wherein the acid comprises at leastone of nitric acid and chromic acid.
 8. The method as recited in claim5, wherein the glass disk substrate is a silicate glass disk substrate,and wherein the step of determining the pH and, if necessary, the stepof adding the acid, is repeated to maintain the adjusted melt at a pH nohigher than about
 11. 9. The method as recited in claim 1, wherein theglass disk substrate is a silicate glass disk substrate, the temperatureof the adjusted melt is high enough for the adjusted melt to be a liquidbut lower than the strain point of the silicate glass disk substrate,and the silicate glass disk substrate is placed in the adjusted melt forat least 1 hour.
 10. The method as recited in claim 1, wherein the glassdisk substrate is an aluminosilicate glass disk substrate, thetemperature of the adjusted melt is about 360° C., and thealuminosilicate glass disk substrate is placed in the adjusted melt forbetween about 2 and 4 hours.
 11. A glass disk substrate microetchedusing the method according to claim 1, wherein the microetched glassdisk substrate has an overall surface microroughness (R_(q)) within arange of about 5 Å to about 7 Å.
 12. A method of adjusting the pH of astrengthening melt to provide an adjusted melt for use in microetchingglass substrates, comprising the steps of: providing a strengtheningmelt; providing an adjusted melt by adding a base to the strengtheningmelt such that the pH of the adjusted melt is sufficient formicroetching a glass substrate having a selected glass composition. 13.The method as recited in claim 12, wherein the strengthening meltcomprises at least one of potassium nitrate, sodium nitrate, silvernitrate, potassium dichromate and sodium dichromate; and wherein thebase comprises at least one of sodium hydroxide, potassium hydroxide,calcium hydroxide, magnesium hydroxide, lithium hydroxide and silver (I)oxide.
 14. The method as recited in claim 13, wherein the glasssubstrate is a silicate glass substrate and the pH of the adjusted meltis no less than about
 10. 15. The method as recited in claim 12, whereinthe glass substrate is a silicate glass substrate, and furthercomprising the steps of: determining the pH of the adjusted melt; addingan acid to the adjusted melt if the pH is determined to be above about11.
 16. The method as recited in claim 15, wherein the strengtheningmelt comprises at least one of potassium nitrate, sodium nitrate, silvernitrate, potassium dichromate and sodium dichromate; and wherein thebase comprises at least one of sodium hydroxide, potassium hydroxide,calcium hydroxide, magnesium hydroxide, lithium hydroxide and silver (I)oxide.
 17. The method as recited in claim 16, wherein the acid comprisesat least one of nitric acid and chromic acid.
 18. A method ofmicroetching glass substrates, comprising the steps of: providing astrengthening melt; providing an adjusted melt by adding a base to thestrengthening melt such that the pH of the adjusted melt is sufficientfor microetching a selected glass composition; placing a glass substratehaving the selected glass composition in the adjusted melt.
 19. Themethod as recited in claim 18, wherein the strengthening melt comprisesat least one of potassium nitrate, sodium nitrate, silver nitrate,potassium dichromate and sodium dichromate; and wherein the basecomprises at least one of sodium hydroxide, potassium hydroxide, calciumhydroxide, magnesium hydroxide, lithium hydroxide and silver (I) oxide.20. The method as recited in claim 19, wherein the glass substrate is asilicate glass substrate, the temperature of the adjusted melt is highenough for the adjusted melt to be a liquid but lower than the strainpoint of the silicate glass disk substrate, and the silicate glasssubstrate is placed in the adjusted melt for at least 1 hour.